Significant progress in optoelectronic technology, including reduction in price and improvement in availability and characteristics of key optoelectronic components such as semiconductor lasers, photodetectors, and position-sensing photodiodes, has created an opportunity for improving detection of sound waves using microphones having optical transducers. Optical transducers offer advantages over the non-optical transducers presently used in microphones, including higher resolution, higher signal-to-noise ratio, immunity to electromagnetic radiation, and greater linearity.
In U.S. Pat. No. 5,262,884 for "Optical Microphone With Vibrating Optical Element," which issued to Jeffrey C. Buchholz on Nov. 16, 1993, an optical microphone is described which includes a vibrating membrane defining a diaphragm for receiving acoustic signals, an optical element, such as a lens, attached to the membrane for vibrating therewith in direct relationship with the acoustic input signals, and fixed fiber optic cables placed in alignment with the lens for directing light from a light source at the remote end thereof toward the lens, and transmitting the directed light from the lens to a detector. Single or dual fiber optical geometry may be used.
The lens may be fabricated by placing a drop of optical epoxy directly on the membrane. The vibrating membrane/lens combination varies the amount of light collected by the fiber optic cable at the acoustic signal frequency in a proportional manner to the strength of the acoustic signal. That is, there is a direct relationship between movement of the lens and the vibration of the membrane in response to the receipt of acoustic signals directed onto the surface of the membrane. The fiber optic cables are fine-tuned to optimize the microphone response.
In U.S. Pat. No. 4,422,182 for "Digital Microphone," which issued to Hideyuki Kenjyo on Dec. 20, 1983, a microphone which generates a digital signal in response to a diaphragm is described. A cylindrical reflecting mirror is integrally attached to the diaphragm and reflects a band-shaped light beam to an array of photoelectric transducers. A binary code pattern is formed on the surface of the mirror which modulates the incident light beam as the relative position of the code pattern and the light beam varies. The modulated light beam is transformed into the digital signal by the array of photoelectric detectors. The binary code pattern consists of a combination of reflecting and non-reflecting areas arranged as four bit words, while the detector comprises an array of four photoelectric transducers because the pattern is a four bit binary code pattern. As the diaphragm moves under the influence of incident acoustic energy, the binary code pattern is scanned by the by the band-shaped light beam, thereby modulating the light beam which is incident on the transducers, whereby the modulated light beam is converted into a digital signal, each transducer being related to respective bits of the binary code. Thus, the binary code output signal designates the amount and direction of the displacement of the diaphragm. In another embodiment of the Kenjyo invention, an aluminum film having the binary code pattern is applied to the light-receiving surface of the transducers. This pattern consists of a combination of light transmitting areas and light absorbing areas.
In U.S. Pat. No. 3,286,032 for "Digital Microphone," which issued to Elmer Baum on Nov. 15, 1966, an earlier microphone for generating a digital code output directly from sound waves is described. A diaphragm intercepts sound waves, and a motion is imparted thereto which is proportional to the amplitude of the sound wave. A plurality of photosensitive devices is arranged in a code matrix, and light from a source thereof is directed through a collimating device having a line configuration onto a mirror suspended from or attached to the diaphragm which reflects the light onto the matrix. A timing generator produces periodic pulses to sample the code matrix. The sampling is achieved by having the code matrix include a plurality of photosensitive devices arranged to be activated by the sampling pulse and to pass or gate an output to the digital outputs when excited by the reflected light.
In the previous two references, direct digital output from the microphone, which is directly related to the displacement of the microphone diaphragm, was believed to be necessary in order to avoid the use of A/D converters in digital recording audio systems for converting analogue sound signals into digital recordings.
In U.S. Pat. No. 5,333,205 for "Microphone Assembly" which issued to Henry A. Bogut and Joseph Patino on Jul. 26, 1994, a microphone assembly is described which includes a movable diaphragm and a linear light gradient device which translates the movement of the diaphragm into a corresponding amplitude of light to be received at a photodetector. That is, light traveling through an optical fiber is directed through an optical conversion means such as a linearly variable density light gradient (optical filter) which is attached to the diaphragm. A linearly variable neutral density filter having a length of approximately the maximum amount of deflection which the diaphragm can undergo is preferred. As the diaphragm is modulated by sound pressure waves, the light gradient moves an equal amount causing different amounts of light to travel to a recovery optical fiber; the light gradient device is moved between the gap formed by the optical fibers causing different amounts of light to pass corresponding to the amount of deflection. The amplitude modulated optical signal recovered by the optical fiber is detected by a photodetector which converts the received light into corresponding electrical signals. The use of a variable attenuation shutter is also described.
In U.S. Pat. No. 2,835,744 for "Microphone" which issued to Francis S. Harris on May 20, 1958 a microphone is described where a light source, a fixed entrance slot for collimating the light from the light source, a detector, and a fixed exit slot for blocking stray light from reaching the detector, are placed on one side of an acoustic-wave sensitive diaphragm. The two fixed slots are aligned such that the light from the light source passes directly through each slot and impinges on the detector. A shutter, having the form of a flat plate of material also having a slot therein, is fastened to the diaphragm in such a manner that it moves therewith, is located between the two fixed slots. When sound waves impinge on the diaphragm, the shutter is displaced, thereby changing the amount of light reaching the detector.
A particularly desirable quality of microphones which have optical transducers is independence from variations in light intensity. Additionally, linearity of response is essential. Although microphone diaphragm technology has evolved such that linearity of motion in response to acoustic input is excellent, none of the above-described references teach the use of linear motion detection systems to take advantage of this technology.
Accordingly, it is an object of the present invention to provide an optical microphone for simultaneously monitoring the spatial and temporal location of light directed onto a diaphragm moving in response to incident sound waves and reflected therefrom.
Yet another object of the present invention is to provide an optical microphone for simultaneously monitoring the spatial and temporal location of light directed onto a diaphragm moving in response to incident sound waves and reflected therefrom, such that the detected signal is independent of the intensity of the light.
Still another object of the invention is to provide an optical microphone for simultaneously monitoring the spatial and temporal location of light directed onto a diaphragm moving in response to incident sound waves and reflected therefrom, such that the detected signal is linearly related to the motion of the diaphragm.
A further object of the invention is to provide an optical microphone for temporally monitoring the intensity of light directed onto a diaphragm moving in response to incident sound waves and reflected therefrom where the reflected light is partially blocked by a fixed edge.
Yet a further object of the invention is to provide an optical microphone for temporally monitoring the intensity of light directed onto a detector and interrupted by a beam stop which follows the motion of a diaphragm moving in response to incident sound waves.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.